Digestive System Flashcards
Digestive System
• Digestion - The breakdown of carbohydrates, proteins & lipids into products small enough to be absorbed into the blood & cells
• The digestive system, or Alimentary Canal, is a continuous tube that runs from the mouth to the anus.
• It is also associated with the pancreas, gall bladder and liver, which are not part of the canal, but play a role in digestion.
• Six activities of digestive system:
Ingestion of food
Mechanical digestion
Chemical digestion
Movement of food along alimentary canal
Absorption of digested food into blood and lymph
Elimination of material that is not absorbed
Alimentary Canal
- Mouth
- Oesophagus
- Stomach
- Small Intestine
- Large Intestine
- Anus
Digestion
There are 2 types of digestion:
Mechanical:
• Physical break up of food into smaller particles
• Eg. Teeth grinding up food
Chemical:
• Chemical break down of complex molecules into their simpler molecules
• Eg. Hydrochloric acid digesting food
Mouth - Mechanical
- Mastication: process of chewing food, breaking it down into smaller particles.
- Teeth (4 types):
- Incisors (4) – chisel shaped teeth for biting or cutting, 4 at front
- Canines (2) – tearing teeth, 1 on each side of incisors
- Premolars (4) – grinding teeth, 2 on each side
- Molars (6) – crushing or grinding teeth, 3 on each side
Mouth - Chemical
• Saliva is secreted by three pairs of Salivary Glands
It contains:
• Mucous: for lubricating the bolus and helping food stick together
• Salivary Amylase: which begins the digestion of starch into polysaccharides or disaccharides (pH 7)
• Substances that kill bacteria in food
Mouth - Other Functions
- Ingestion of food
- Saliva dissolves food so taste receptors can be stimulated
- After chewing food is formed into a round lump called a Bolus by the tongue and is pushed towards the pharynx for swallowing
Oesophagus
- As the food moves through the pharynx, the epiglottis closes off the trachea that leads to the lungs
- The oesophagus carries bolus from pharynx to stomach. It passes through the diaphragm which is a sheet of muscle that separates the chest cavity from the abdominal cavity.
- The movement of food is lubricated by mucous
- Two layers of muscle surround the oesophagus:
- Longitudinal muscle – runs along the length
- Circular muscle – arranged in circles around canal
- The bolus is pushed along the oesophagus by a wave of contraction of the circular muscles known as the process of Peristalsis
Stomach - Mechanical
- Waves of muscular contraction in the stomach wall
- Three layers of muscle surrounding the stomach:
- Circular, longitudinal, and additional oblique layer
- Waves of muscular contraction of the stomach wall allow the stomach to contract in a variety of ways to churn food.
- Food becomes a thick soupy liquid called Chyme
Stomach - Chemical
• Gastric Juice secreted by Gastric Glands
Contains:
Hydrochloric Acid:
• Destroys bacteria
• Provides optimum pH for Pepsin
Mucus:
• Prevents stomach digesting itself
Pepsinogen:
• Pepsinogen is the inactive form of the digestive enzyme Pepsin (Gastric Protease)
• When Pepsinogen comes into contact with HCl acid it is converted to active form of Pepsin.
• Pepsin breaks down proteins -> polypeptides
• optimum pH in acidic conditions (pH 2)
Stomach - Other Functions
- Mucosa is the lining of the stomach and is specialised for the secretion of gastric juice.
- The gastric glands which secrete gastric juice are located in narrow tube-like structures called gastric pits.
- The HCl acid, mucus and pepsin are all secreted by different cells in the gastric pits.
- Pyloric Sphincter – a thickening of circular muscle at the bottom of the stomach, which controls the flow of material into the duodenum (start of small intestine).
- The constriction is enough the prevent stomach contents flowing into duodenum, unless pushed along by peristalsis
- Contents are usually pushed through after 2-8 hours
- No nutrients are absorbed through the stomach walls due to the thick layer of mucus.
- However, some alcohol and drugs such as aspirin are absorbed in the stomach.
Small Intestine
• Small intestine is 6m long and is made up of 3 sections:
Duodenum: receives material from stomach and continues digestion
Jejunum
Ileum
Small Intestine - Mechanical
- Circular muscle contraction called Segmentation
- Breaks up bolus further and brings it into contact with lining for absorption
- Mixes contents with juices and bile
- The Liver produces a substance called Bile
- The liver secretes bile to the Gall Bladder, where it is stored and concentrated
- Bile enters the Duodenum through the Common Bile Duct
- Bile contains Bile Salts which act like a detergent and emulsify lipids, by breaking them into tiny droplets.
- This increases the surface area in which lipases can work on the lipids to break them down.
- Bile contains no digestive enzymes, so it is considered mechanical not chemical digestion.
Small Intestine - Chemical
• The Pancreas produces and secretes Pancreatic Juice, which is released into the Duodenum from the Pancreatic Duct.
• It contains 4 enzymes:
Pancreatic Amylase – starch/polysaccharides -> disaccharide
Pancreatic Protease (aka Trypsin) – proteins/polypeptides -> dipeptides
Pancreatic Lipase – lipids -> fatty acids and glycerol
Pancreatic Ribonuclease and Deoxyribonuclease – digests RNA and DNA
- The Pancreas produces and secretes Pancreatic Juice
- Pancreatic juice is basic in pH so it works to neutralise any acid that comes through from the stomach.
- Further down the small intestine, glands in lining secrete
• Intestinal Juice, which contains 3 enzymes:
Intestinal Amylase – disaccharides -> monosaccharides
Intestinal Peptidase – dipeptides -> amino acids
Intestinal Lipase – lipids -> fatty acids and glycerol
Small Intestine - Absorption
• The small intestine is the location of absorption of the products of digestion.
• It has an enormous Surface Area to allow for efficient absorption, which is achieved by:
Long length – 6m
Folding of inner lining (called Mucosa)
Villi and microvilli
- Each villus is 1mm long and covered by a single layer of cells which means the nutrients only have to travel a very small distance into the blood vessels.
- Each villus contains a single lymph capillary called a Lacteal, and each lacteal is surrounded by a network of blood capillaries.
- Each villus is also covered by many Microvilli, which are microscopic projections that increase the surface area even further.
- Absorption is assisted by muscular contractions in the small intestine that keep the contents moving along to maintain the concentration gradient (difference).
- The type of transport of the nutrients into the villi depending upon the type of nutrient:
- Monosaccharides (eg. Glucose) – Active transport into the blood capillaries
- Amino acids – Active transport into blood capillaries
- Water, minerals and water-soluble vitamins – osmosis into blood capillaries
- Fatty acids and glycerol – Simple diffusion into the cells of the villi where they recombine and then enter the lacteal
- The substances that enter the blood capillaries travel by the Hepatic Portal Vein to the liver.
- From here they are either removed for further processing or remain in the blood to be carried to other cells.
- Fatty acids and glycerol, along with any fat-soluble vitamins enter the lacteals.
- They are transported by the Lymph System which eventually empties into the blood through lymph veins in the upper chest.
Large Intestine
- 1.5m long, but larger in diameter than small intestine
- Small intestine joins to large intestine at the Caecum, a small pouch 6cm long
- The Appendix is a small tube attached to Caecum; it has no function
- The Colon is the longest part and is divided into the Ascending, Transverse and Descending sections.
- Contents of intestine are pushed into the rectum by peristalsis. As the walls stretch, they trigger Defaecation
- The anus is the external opening at the end of rectum. Surrounding it is circular muscle called the anal sphincter that controls movements.
- No digestion happens in the Large Intestine so no enzymes are present
- 18-24 hrs for material to pass through
- Absorbs any water that is remaining in digestive tract -> contents become more solid (faeces)
- Bacteria break down any remaining organic compounds
- Vitamins produced by bacteria and minerals also absorbed
- Faeces contains water, undigested food (mainly cellulose), bacteria, bile pigments (give colour), and remains of cells from canal lining.
- Defaecation (Elimination): the removal of undigested waste material from the digestive tract via the anus
